I do not think we will gain enough information to actually map much
history, but these certainly are indicator minerals. At least we now
have an expectation that can be confirmed with bore holes.
We have a whole range of gem like materials that do tell geological
stories. It is through them that we zero in on diamond mines. Add
to that our understanding now of the genesis of mineralogy through
temperature gradients and one almost begins to think we have a
chance. Unfortunately most of it is still just rock.
Otherwise it is a nice addition to our understanding.
In Glittering Gems,
Reading Earth’s Story
CARL ZIMMER
Published: June 13,
2013
A jewelry store is an
archive of the Earth. Every gem fixed to every ring or necklace was
forged deep inside our planet, according to its own recipe of
elements, temperature and pressure.
But it has taken a
while for geologists to decode the cookbook for gems. Jade, for
example, puzzled geologists for decades. “For a long time people
looked at this crazy rock, and it didn’t make any sense,” said
George Harlow, a geologist at the American Museum of Natural History.
But thanks to the research of Dr. Harlow and other geologists, jade
now has a back story: It formed in dying oceans.
The discovery of gems
like rubies and jade thus signifies more than just a new supply of
bling in jewelry stores. It tells geologists some important things
about the planet.
If rocks contain
jade, the scientists can be fairly sure those rocks are a vestige of
an ocean buried underground. Rubies, on the other hand, appear
in places where mountains formed from continental collisions, even if
those mountains were eroded away millions of years ago.
Gems may thus preserve
precious clues to some of the most profound questions about the life
of our planet.
The Earth is covered
with tectonic plates. In some places, like off the coast of the
northwestern United States, ocean plates are getting pushed
underneath continental ones. As the plates sink, they are squeezed to
titanic pressures. They also get cooked. Under these extreme
conditions, their atoms are combined into new molecular arrangements
that would never emerge anywhere else on Earth. Hot jets of
mineral-rich fluid rise from the plates, pushing up into the
overlying rock. Among the things that emerge from that fluid is a
mixture of sodium, aluminum, silicon and oxygen known as jadeite —
a form of jade.
In the current issue
of the journal Earth and Planetary Science Letters, Dr. Harlow and
his colleagues report new findings that support this chain
of events. At the bottom of the Caribbean Sea, they found
jade-bearing deposits that are 30 million years older than the
surrounding rock. That’s what you would expect if jade-producing
fluid rose up from sinking oceanic crust long before other material
from a sinking ocean plate.
In the journal
Geology, Dr. Harlow — writing with Robert J. Stern of the
University of Texas at Dallas, Tatsuki Tsujimori of Okayama
University in Japan and Lee A. Groat of the University of British
Columbia — explores some stories that gems like jade can tell. Each
is different. While jade is produced from dying oceans, for example,
rubies are forged in newborn mountains.
Some mountain ranges,
like the Himalayas, were formed when two continental plates
collided. Fifty million years ago, the Indian subcontinent was
an isolated island. In a slow-motion crash, it plowed into Asia. The
force of the impact crumpled rocks for hundreds of miles inland,
producing mountains.
As mountains rose
overhead, the Indian plate slid underneath Asia. Once again, the rock
was squeezed and heated. Continents are ringed by rocks like shale,
formed from sediments washing off of land. When crushed in this
subterranean forge, shale can produce crystals of aluminum and
oxygen.
If these crystals stop
developing, they become sapphires. But the crystals may instead get
pushed up toward the surface of the Earth. The overlying rock they
move into is rich in chromium. The chromium atoms push the aluminum
atoms out of the crystals and take their place, giving them a red
color. “When they get a little chromium in them, we call them
rubies,” Dr. Stern said.
The oldest deposits of
jade and rubies date to only about 600 million years ago. The Earth
itself is more than 4.5 billion years old, which naturally raises the
question of why there aren’t any jade stones or rubies from the
first nearly four billion years of the planet’s existence.
Dr. Stern, for one,
thinks the answer is that plate tectonics — the stirring of the
planet’s interior that drives plates around its surface — didn’t
exist for most of Earth’s history. “I would say it started less
than a billion years ago,” he said.
That’s a
controversial idea in the geological community. Perhaps gems may
someday resolve it.
I thought this was very interesting. I love gemstones but know little of their origin & evolution. Thanks.
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